i) Field of the Invention
This invention relates to a method and apparatus for reacting a gas having a low solubility in water with a substance in an aqueous medium.
ii) Description of Prior Art
Certain gases such as oxygen have a low solubility in water and aqueous liquids, for example, solutions and suspensions. The solubility of oxygen, in an aqueous liquid, decreases as the temperature of the aqueous liquid increases and as the concentration of the substance dissolved or suspended therein increases. This is illustrated in Table I below.
______________________________________ TEMPER- PRESSURE ATURE SOLUBILITY LIQUOR ATMOS. .degree.C. MG/L ______________________________________ WATER 1 20 44 WATER 7 93 143. NaOH SOLN 7 93 57 2M ______________________________________
In many industrial operations it is required to react a large quantity of oxygen gas with a substance in an aqueous vehicle, for example, a solution or suspension of the substance in water.
When the ratio of gas to liquid is very high, the method conventionally employed involves dissolving the gas, for example, oxygen, in the liquid by sparging into an agitated tank holding a large volume of the liquid, or the use of a tall gas absorption tower. However, these conventionally employed methods are not especially cost effective and involve large apparatus which is space consuming.
Furthermore, sparging into an agitated tank open to the atmosphere results in low oxygen transfer and poor utilization of oxygen. Sparging into a closed and pressurized tank requires a gas recirculation system which increases the cost significantly.
Packed absorption towers have been used, with both gas and liquid recirculation. However, such towers require long gas-liquid contact times with high gas and liquid recirculation rates necessitating large towers and high capital cost.
U.S. Pat. No. 3,696,929, I. S. Shah, issued Oct. 10, 1972, describes a low pressure sinuous pipe for oxidation of black liquor with oxygen, in which turbulence is low so as to inhibit foam formation, and the concentration of free oxygen in the dispersed bubble phase remains substantially constant. The U.S. Patent does not address the problem of efficient oxygen utilization based on the low solubility of oxygen in the liquor being oxidized. U.K. Published Patent Specification 2,006,852, published May 10, 1979, B. S. Kirk et al describes gravitational fall zones for injection of oxygen into pulp and controlled introduction of alkali to maintain an alkaline pH of 10-12.5; provision is made for venting excess, undissolved oxygen.